One of many euphemisms for "masturbation" (e.g. slappin' the salami, jack off, beat off, buffing the bishop, glean the obscene bean, choking the chicken, spanking the monkey, waxing the wonker, greasing the guppy, shooting putty at the moon, and Rosie Palmer and her Five Sisters doing the Four Knuckle Shuffle.

Frank will be a little bit late. He's still back at his house jackin' the beanstalk.